Turbochargers adapted to driving an exhaust turbine, utilizing exhaust energy of an engine, to by turn drive a charging compressor are known as a method for compressing air and causing the engine to take in compressed air. Since such a turbocharger utilizes exhaust gas of an engine, the flow rate of exhaust gas is reduced and hence the exhaust turbine revolves at a low rpm in a low rpm range of the engine so that it is practically impossible for the charging compressor to operate effectively. A method of making the nozzle area of the turbocharger variable is known to improve its performance. Such a turbocharger is called a variable geometry turbocharger.
In variable geometry turbochargers, movable nozzle vanes are arranged in the nozzle of the exhaust turbine and the nozzle area (the nozzle aperture area) is regulated by driving the nozzle vanes to swing. Since the displacement volume of the engine is small in a low rpm range of the engine, the flow rate of exhaust gas flowing into the exhaust turbine is raised by driving the nozzle vanes to swing so as to reduce the nozzle area and increase the rotational energy of the exhaust turbine wheel. In this way, it is possible to raise the supercharging capacity of the charging compressor.
Japanese Patent Laid-Open Publication No. Hei.11-343857 discloses such a known variable geometry turbocharger. Referring to FIG. 6 of the accompanying drawings, in a variable geometry turbocharger disclosed in the above-cited patent document, shafts 81 of a plurality of nozzle vanes 80 (indicated by dotted lines in FIG. 6) that are arranged on a stationary board 82 are fitted to the stationary board 82 so as to run through the board 82 and an actuator ring 83 is rotatably arranged at the outside (outer peripheral side) of the fixed board 82. The shafts 81 projecting from the stationary board 82 and the actuator ring 83 are linked to each other by levers 84. More specifically, each of the levers 84 is secured at an end thereof to the corresponding one of the shafts 81 of the nozzle vanes 80 and at the other end thereof to the corresponding one of fitting shafts 83B of the actuator ring 83 so as to be engaged with the shaft 83B and able to swing.
A ring-shaped support member 85 is arranged outside the above-described actuator ring 83 and provided with a supporting shaft 85A projecting from the periphery of it. A link arm 86 is supported by the supporting shaft 85A so as to be able to swing. The link arm 86 is connected at an end thereof to a projection 83A arranged on the actuator ring 83 and at the other end thereof to an actuator rod 87 adapted to turn a link arm 86 by way of respective oblong holes 86A, 86B.
With the above-described arrangement, as the actuator rod 87 is driven to axially reciprocate, the link arm 86 swings around the supporting shaft 85A so as to turn the actuator ring 83 connected to the corresponding end thereof.
As the actuator ring 83 turns, the levers 84 fitted to the actuator ring 83 also turns so that the shafts 81 also turn with them. As a result, the nozzle area can be regulated by changing the swinging angle of the nozzle vanes 80 whenever necessary.
Meanwhile, to improve the rotational efficiency of the exhaust turbine, the housing of the exhaust turbine is required to be effectively sealed for the purpose of efficiently supplying exhaust gas to the turbine wheel.
However, with the above-described variable geometry turbocharger, one of the opposite ends of the link arm 86 is located inside the exhaust turbine while the other end is located outside the exhaust turbine and driven to swing in this condition. Therefore, the housing covering the exhaust turbine needs to be provided with a sufficiently large slit that allows the link arm 86 to swing. Then, the housing is accompanied by a problem that it is difficult to maintain the effectively sealed condition of the slit for a long period of time.
Additionally, for the nozzle vanes 80 to be driven to swing, the linear motion of the actuator rod 87 needs to be converted into a rotary motion by means of the actuator ring 83 located inside the housing so that the link arm 86 has to be provided with oblong holes 86A, 86B to make the mechanism for transmitting the driving force a complex one.